silicone does not adhere well to acrylic. it will work fine for stuff like dividers and the like where the water pressure is pretty much equal on both sides. the silicone will adhere fine to the glass sides of the tank. this is one of those things where siliconing an acrylic divider into a glass tank will work, but not the other way around. Like Jay has been saying it can be done reliable if the pressure is helping you, overflows in the corners is a great example. the pressure of the water is forcing the acrylic agains the glass and against the silicone.
FYI-100% silicone also does not stick worth a flip to PVC. it will seperate over time and just peel right off. can not tell you how long it took me to get that through my thick head.

some silicone adhesives will work, but they are not 100% silicone. 100% silicone is what is recommended for aquarium repairs.
G~